Robots have been deployed across numerous industrial and manufacturing environments to promote reliability and cost savings. Robotic arms that are used to move a work tool between locations are typically driven by rotating motors via low-friction gearboxes. The low-friction gearboxes convert rotational motion with a high output efficiency, but generally permit the motor to continue moving for a significant time following an emergency stop or when power is interrupted. Additionally, the low-friction gearboxes may lead to situations where a raised robotic arm in a static position falls under gravity during the emergency stop or power failure and potentially cause damage to equipment or harm to humans. As a result, motors for robotic arms may be equipped with an emergency-stop brake to avoid these hazards.
Conventionally, one type of emergency-stop brake utilizes an electromechanical friction device with spring loading; the position of the friction device is controlled by a solenoid. During an emergency stop or loss of power, the current stops flowing through the solenoid, causing engagement of the brake and thus stopping the motor. These electromechanical brakes, however, generally “lock” the motor within a very short time and may trap a human operator located in spatial proximity. Although a secondary safety circuit may be used to release the brake, harm to the operator may result before release occurs. Furthermore, because the secondary safety release circuit is often battery powered, the reliability thereof is decreased. In addition, the electromechanical motor brake and/or the secondary safety circuit add weight and cost to the robotic system.
Another braking approach is to short-circuit the electric motor during an emergency or power failure; the short-circuited motor gradually slows down with limited mechanical wear. This approach may reduce the risk of trapping a human operator and thus avoid using a secondary safety circuit to release the brake. However, an extra independent power source—for example, a battery—is typically required to power a control logic circuit for controlling and supporting the braking circuit (or to short-circuit the motor directly) during an emergency stop or loss of power. Maintenance and the additional cost of the extra power source present a disadvantage of utilizing this approach.
Consequently, there is a need for motor brakes that can gradually reduce the speed of a motor during an emergency stop or power failure to prevent human entrapment without the need for an extra independent power source.